Question

[Dương Hà Linh]

Các bn ghi rõ cách giải nhé mk đang cần gấp lắm ai nhank mk tick cho cảm ơn

1: Tính nhanh

A= 1/1*3+1/3*5+1/5*7+1/7*9+1/9*11

B= 1/1*3+1/3*5+1/5*7+...+1/2017*2019

C= 7/1*3+7/3*5+7/5*7+...+7/2017*2019

Answer 1

[Đàm Thị Minh Hương]

\(2A=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}=\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\)

        \(=1-\frac{1}{11}=\frac{10}{11}\)

\(\Rightarrow A=\frac{5}{11}\)

Answer 2

[Đàm Thị Minh Hương]

\(2B=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{2017.2019}=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2017}-\frac{1}{2019}\)

        \(=1-\frac{1}{2019}=\frac{2018}{2019}\Rightarrow B=\frac{1009}{2019}\)

\(\frac{2}{7}C=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{2017.2019}=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2017}-\frac{1}{2019}\)

           \(=1-\frac{1}{2019}=\frac{2018}{2019}\Rightarrow C=\frac{2018}{2019}:\frac{2}{7}=\frac{7063}{2019}\)